1. Field of the Invention
The present invention relates to a Raman amplification repeater using the amplification function of a signal light caused by Raman scattering which occurs when a pumping light LD is applied to a transmission path fiber and, more particularly, a Raman amplification repeater which enables control of Raman amplification.
2. Description of the Related Art
For meeting an increasing demand for communication created as the Internet has been widely used, transmission capacities of optical transmission systems forming a basic communication network have been recently increased at a surprising speed.
In order to cope with such a rapid increase in transmission capacity, optical wavelength division multiplexing techniques (WDM technique) have been established to enable transmission data capacities to be increased. In a long-distance transmission system, however, expansion of a repeating interval is a target which is hard to achieve as well as scale-up of a transmission capacity.
Under these circumstances, for ensuring a signal to noise power ratio (optical SN ratio) per one wavelength and mitigating transmission waveform distortion caused by the fiber nonlinear effect, proposed is a transmission method, called Raman amplification, of canceling a loss of a transmission path. In this method, in order to realize long-distance transmission, required is flat gain characteristics as well as reduction in the fiber nonlinear effect and reduction in noise.
Structure and operation of a conventional repeater using Raman amplification will be described with reference to FIG. 8.
In FIG. 8, wavelengths of pumping light LDs (Laser Diode) 13a, 13b and 13c are 1462.4 nm, 1475.0 nm and 1503.1 nm, respectively, and a signal wavelength band ranges from 1574 to 1609 nm. After being multiplexed by WDM (Wavelength Division Multiplex) couplers 12a and 12b, pumping lights are multiplexed by a pumping light WDM coupler 11 with a signal light on an optical transmission path in the reverse direction to each other. Pumping light output from a pumping light output point onto the optical transmission path amplifies, in the optical transmission path, a signal light band which is about 13.2 THz apart from the pumping light.
First, by using an appropriate transmission path fiber, obtain output power of the pumping light LDs 13a, 13b and 13c having different wavelengths so as to have a flat gain spectrum after, for example, 10 dB Raman amplification.
Next, make connection to an actual transmission path fiber and operate the respective pumping light LDs 13a, 13b and 13c by the pumping light output power obtained before the connection to the transmission path to conduct Raman amplification. FIG. 9 shows a gain spectrum obtained when Raman amplification is conducted by the above-described conventional Raman amplification repeater.
As shown in FIG. 9, with the Raman amplification repeater having such a structure as shown in FIG. 8, even when the output power of the pumping light LDs 13a, 13b and 13c having different wavelengths is obtained so as to have a flat gain spectrum after Raman amplification, it is difficult to keep the gain spectrum within a signal band be flat because of a difference in transmission path fibers or in intra-office losses or the like.
In order to solve such a problem, the wavelength characteristic control method of optical transmission power by Raman amplification disclosed in Japanese Patent Laying-Open (Kokai) No. 2001-7768 (Literature 1), for example, shows a technique of controlling gain wavelength characteristics of Raman amplification by monitoring wavelength characteristics of optical transmission power as of after Raman amplification by two PDs (Photo Diode). Use of such a method aims at keeping a gain spectrum be flat by mitigating effects produced by a difference in transmission path fibers or in intra-office losses or the like.
As described in the foregoing, conventional Raman amplification repeaters have the difficulty in keeping a gain spectrum within a signal band be flat due to a difference in a transmission path fiber or in intra-office losses or the like.
On the other hand, the wavelength characteristic control method of optical transmission power by Raman amplification as disclosed in Literature 1, which controls only an inclination of a wavelength spectrum by monitoring only the opposite sides of the wavelength spectrum or controls only a wavelength at the top (a part having high power) of the wavelength spectrum, has a shortcoming that flatness of an output spectrum can not be satisfactorily maintained in a case, for example, where a signal light whose spectrum has an inclination is applied.